This research will study patients who are experiencing an acute rejection of a transplanted kidney. Participants will receive an investigational new medication, HuM291, that will be given intravenously (IV). The purpose of this study is to examine the effect and safety of different amounts of HuM291 given in 3 doses to kidney transplant recipients experiencing a mild to moderate rejection. The length of participation in this study will be 104 days. This medication is a humanized monoclonal antibody that is made in a laboratory. Antibodies are proteins made by the body that help us fight infection or invasion by foreign substances. Antibodies work by clamping onto germs or invading substances. Normally our bodies do not make antibodies that clamp on, or bind to our own tissues, but mice can produce antibodies that bind to human tissues. Mouse antibodies can be changed in a laboratory so that they are almost exactly like human antibodies. This type of antibody is called a "humanized monoclonal antibody." HuM291 is a humanized monoclonal antibody that is made in a laboratory. HuM291 binds to parts of T cells and causes them to decrease their function. This has the effect of lowering immunity but it may also help reverse rejection of the kidney.